This invention relates to a piston ring, particularly an oil scraper piston ring of axially small height formed of an expander spring made of sheet material and two laminar rings which, on the one hand, are axially supported on a plurality of circumferentially arranged, radially extending tongues of the expander spring and, on the other hand, are radially supported by legs which are approximately axially bent from the tongues of the expander spring. The legs have radially inwardly oriented extensions. The tongues and leg of the expander spring are formed by a circumferential series of slots cut into the expander spring alternatingly from the radially inner and radially outer edge thereof.
It is generally known to provide oil scraper rings having a multi-part steel ring construction, wherein for radially pressing at least one laminar ring against the cylinder wall, there is provided an expander spring made of sheet material. The expander spring is slotted in such a manner that a highly flexible construction is obtained which may be bent into a ring of defined cross-sectional configuration to provide a suitable solution for the selected engine.
Conventionally, the slots are provided by stamping the sheet material in a given single direction. U.S. Pat. No. 3,633,260 discloses an expander spring ring made with such a process. If such a stamped-out strip is bent into its permanent shape, for example, a shape wherein a bending angle of approximately 180.degree. is required, as disclosed, for example, in German Patent No. 3,530,175, fissures appear in the ring, in the zones of tension derived from the bending stresses to which the strip material is exposed. Such fissures, whose appearance is caused to a great measure by the burrs resulting from the stamping operation, start from the cut edge of the expander spring. The stamped cross section is composed, as viewed in the direction of stamping, of approximately 2/3 of a smooth cutting face and 1/3 of tensioned (fissured) face. This ratio is dependent from a great number of parameters, such as the type of the sheet material and the sharpness (state of wear) of the cutting tool. At the end of the tension-stressed surface stamping burrs appear which, during the subsequent bending of the metal strip cause fissures at those locations. The fissures adversely affect the service life of the expander spring and thus also shorten the service life of the engine served by the piston rings.